One of techniques for preserving foods, pharmaceutical products and the like is preservation utilizing oxygen absorbing agents (oxygen scavengers). Specifically, the preservation utilizing oxygen absorbing agents is a technique in which an oxygen scavenger capable of absorbing oxygen in atmosphere, together with an object, is placed within a hermetically sealable packaging body, and the inside of the hermetically sealable packaging body is brought to an oxygen-free state to prevent an oxidation-derived deterioration, a fungal deterioration, discoloration and the like of objects.
Oxygen scavengers formed of various inorganic materials and oxygen scavengers formed of various organic materials have hitherto been proposed for the removal of oxygen in atmosphere. Examples thereof include oxygen scavengers comprising inorganic main agents, for example, iron or other metal powders, sulfites, bisulfites, and dithionite, and oxygen scavengers comprising organic main agents, for example, L-ascorbic acid, erythorbic acid and salts thereof, saccharides such as glucose, and reducing polyhydric alcohols such as cathecol and pyrogallol.
These conventional oxygen scavengers, however, suffer from a problem that, in use, when a material that can supply water or moisture does not exist, an oxygen scavenging capability high enough to be used for practical use cannot be provided. Specifically, in conventional oxygen scavengers, an oxygen scavenging capability high enough to be used for practical use cannot be provided without mixing of the material with water or moisture retained thereon, for example, compounds containing water of crystallization, in use, or utilization of water vapor released from foods or the like to be preserved. Accordingly, difficulties have been encountered in applying conventional oxygen scavengers to pharmaceutical products or dried foods that should be used or preserved under drying conditions, or to storage of metal products that are weak against water or moisture without rusting.
Accordingly, oxygen absorbing agents that do not require moisture in oxygen absorption have been demanded in these applications. Oxygen absorbing agents reported as meeting this demand include, for example, (a) oxygen scavengers comprising cerium oxide utilizing oxygen defects as a main agent (Japanese Patent No. 4001614 (Patent Document 1)), (b) oxygen scavengers comprising titanium oxide containing oxygen defects as a main agent (Japanese Patent No. 4248986 (Patent Document 2)), (c) oxygen scavengers comprising a metal subjected to hydrogen reduction as a main agent (Japanese Patent Application Laid-Open No. 277148/1987 (Patent Document 3), and (d) oxygen scavengers that utilize autoxidation of organic substances.
Among the above oxygen scavengers, oxygen scavengers disclosed in the above (a) and (b) utilize rare metals as starting metals that are rare and expensive. Further, the rare metals should be obtained through import from foreign countries, and, thus, depending upon further situation development, there is a possibility that the purchase of stock is varied, making it impossible to provide stable amount of production. Accordingly, these oxygen scavengers are not always satisfactory from the viewpoints of cost and stable supply of the rare metals. The oxygen scavenger disclosed in the above (c) requires the provision of a large hydrogen reduction equipment in the production thereof and thus cannot be simply produced and, at the same time, cannot be said to have good handleability in the atmosphere. Furthermore, the oxygen scavenger in the above (d) utilizes an oxidation reaction of the organic substance as the main agent, posing a problem of a by-product produced after oxygen absorption.
Accordingly, there is still a demand for an oxygen absorbing resin composition that can absorb oxygen in the atmosphere even in a moisture-free or substantially moisture-free atmosphere, is advantageous in stable availability of starting materials at relatively low cost, is substantially free from the problem of the by-product, and is not necessary to provide a large apparatus for hydrogen reduction as an incidental equipment.
Techniques have recently been adopted in which objects to be stored are packaged with sheet-shaped packaging materials obtained by kneading an oxygen scavenger with a resin and forming the kneaded product into a sheet or film (for example, Japanese Patent No. 3496427 (Patent document 4) and International Publication WO 2010/147097 (Patent document 5)). Further, the use of sheet-shaped packaging materials has also been desired for storage of pharmaceutical products or dried foods that should be used or stored under dried conditions, and rust preventive storage of metal products in which the presence of water or moisture is not favorable. Specifically, sheet- or film-shaped oxygen absorbing packaging materials have also been desired to have a capability of absorbing oxygen in an atmosphere, even under a moisture-free or substantially moisture-free atmosphere.
Various techniques have been proposed for storing foods and pharmaceutical products. Hollow containers composed mainly of polyesters such as polyethylene terephthalate (PET) having a high gas barrier capability are used, for example, in teas, fruit beverages, and carbonated beverage. Further, hollow containers produced using gas barrier resins exemplified by polymethaxyleneadipamide resins (hereinafter referred to also as “MXD6 resin”) and ethylene-vinyl alcohol copolymer resins (hereinafter referred to also as “EVOH resin”) are extensively utilized for storage of foods and pharmaceutical products.
Hollow containers of polyester resins used particularly in beverages having a rapidly grown demand have a tendency toward a size reduction from the viewpoint of portable convenience, and various hollow containers having an inner volume of less than 500 mL are used for various contents such as green teas, sports drinks, carbonated beverages, teas, fruit beverages, coffees, and vegetable beverages, and most of these contents are likely to be adversely affected and deteriorated by light or oxygen.
Regarding applications other than beverages, there is an increasing tendency toward the adoption of hollow containers of polyolefinic resins having a small inner volume of about 100 mL in tablet gums, cosmetics, functional supplements, and tablet-type pharmaceutical products. In hollow containers, the proportion of the surface area per unit volume increases with a reduction in size. Accordingly, the quality of contents is deteriorated at a higher speed, and the quality guarantee period is likely to be shortened.
For this reason, various techniques have been proposed and implemented for extending the best-before date of the contents in hollow containers. Japanese Patent Application Laid-Open No. 320662/2002 (Patent document 6) describes, as one example of fundamental techniques for extending the best-before date of the contents of hollow containers, a technique in which a hermetically sealable container having a good oxygen permeability is used in combination with a small bag-shaped oxygen scavenger to suppress an oxidative deterioration of easily oxidizable drugs. This invention, however, has problems to be solved, for example, that, when contents of the hollow container are liquid, a small bag-shaped oxygen scavenger cannot be sealed in the hollow container; a machine for sealing of the oxygen scavenger should be installed; the number of process steps in the production is increased; and the amount of wastes after use is large.
Japanese Patent Publication No. 1824/1987 (Patent document 7) describes, as another example of fundamental techniques for extending the best-before date, a technique on oxygen scavenging multilayered structures such as bags and bottle tubes that include an oxygen absorbing layer formed of a material obtained by incorporating an oxygen scavenger composed mainly of a reducing iron in an oxygen permeable resin. The method proposed in this invention, however, is not simple because a large-scale hydrogen reduction equipment should be provided in combination with iron oxide as a starting material. Further, water is necessary for the oxidation reaction, and contents that can be stored in the multilayered structure are disadvantageously limited to aqueous materials.
For example, Japanese Patent No. 4501044 (Patent document 8) describes, as a further example of fundamental techniques for extending the best-before date, a technique that an oxidizable polyamide resin and a transition metal catalyst are used to impart a high level of barrier capability through absorption of oxygen being permeated from the outside of the hollow container into the inside of the hollow container with the elapse of time and, at the same time, to absorb oxygen dissolved in a liquid contained in the hollow container through the inner wall of the container, thereby suppressing and reducing an oxidation-derived deterioration of the contents. The claimed advantage of this invention is that the hollow container is suitable for use in applications where beers and teas are placed in hollow containers. This invention, however, is disadvantageous in that, due to the utilization of an oxidation reaction of an organic material, by-products derived from the oxidation reaction are produced and an oxidation reaction-derived lowering in strength of the hollow container occurs.